1. Field of the Invention
The present invention relates to an in-line optical isolator used to cope with optical feedback in a high-power fiber laser which is used in laser processing and the like and has a wavelength near 1 μm (=1000 nm).
2. Description of the Prior Art
In a laser such as a semiconductor laser used in optical communication or a fiber laser used in laser processing and the like, when light reflected by optical surfaces or machined surfaces outside a laser resonator returns to a laser element of a semiconductor laser or a fiber laser, laser oscillation becomes unstable. Further, unstable laser oscillation may cause signal noise in the case of optical communication, and may cause a breakage of the laser element in the case of a material processing laser. Accordingly, for the prevention of the reflected optical feedback returning to the laser element, an optical isolator including a Faraday rotator, a polarization separation element, and the like is incorporated into a device including the laser element.
As a Faraday rotator used in an optical isolator for a high-power laser, a terbium gallium garnet single crystal (hereinafter referred to as TGG) or a terbium aluminum garnet single crystal (hereinafter referred to as TAG) has heretofore been used.
However, the TGG and TAG have small Faraday rotation coefficients per unit length. Accordingly, to obtain a polarization rotation angle of 45 degrees so that the TGG or TAG may function as an optical isolator, a long optical path length is needed. Thus, a crystal having a length of not less than 2 cm must be used. Moreover, to obtain a high optical isolation, a strong uniform magnetic field needs to be applied to the crystal, and thus a large and strong magnet is used. This increases the size of the optical isolator, and causes a problem in terms of size when the optical isolator is used in a device including a fiber laser (laser element) in which compact implementation is desired. Moreover, since the optical path length is long, the beam shape of a laser may become distorted in the crystal, and an optical system for correcting the distortion may be needed. Furthermore, since the TGG crystal, the magnet, and peripheral optical components are also expensive, a compact and inexpensive optical isolator has been demanded.
On the other hand, a Faraday rotator made of a bismuth-substituted rare-earth iron garnet crystal film (hereinafter also referred to as a BIG film), which is mainly used in the field of optical communication, has a Faraday rotation coefficient per unit length significantly larger than those of the TGG and TAG. Accordingly, an optical isolator can be greatly miniaturized. However, in a BIG film, in the case where the wavelength of light used is as short as approximately 1 μm (=1000 nm) such as used in a material processing laser, the following phenomenon is known to occur: iron ions strongly absorb the light, absorption further increases due to a temperature rise in the BIG film caused by the absorption, and this causes degradation in the performance of the BIG film.
As a method for solving the problem due to a temperature rise in the BIG film, a method has been proposed in which a temperature rise in the BIG film is prevented by bringing a sapphire heat sink substrate into contact with the BIG film (see Japanese Patent Application Publication Nos. 2005-043853 and 2007-108344).
In the case where an in-line optical isolator is incorporated into a device including a fiber laser used in laser processing and the like, the output side of the optical isolator may be connected to a fiber amplifier (including an optical fiber doped with a rare earth element such as Yb). In this case, the aforementioned optical feedback includes not only light having a wavelength equal to the oscillation wavelength but also amplified spontaneous emission light (referred to as ASE) from the Yb-doped fiber (fiber amplifier), and the power of the optical feedback may be as high as 50% or more for a forward input.
The ASE does not have a sharp wavelength spectrum like that of a laser, but has a wavelength distribution ranging from 1000 nm to 1100 nm with a peak between 1030 nm and 1040 nm. On the other hand, the BIG film shows strong absorption for wavelengths near 1 μm (=1000 nm) as described previously, and the amount of the absorption sharply increases when the wavelength decreases from 1100 nm to 1000 nm.
The BIG film shows strong absorption for wavelengths shorter than 1040 nm when ASE returns from the fiber amplifier to the optical isolator, due to the wavelength dependence of the absorption of the BIG film. This causes a temperature rise in the BIG film. Accordingly, there has been the problem that an increase in the forward insertion loss of the in-line optical isolator and the isolation degradation thereof occur.
The present invention has been made in view of such problems, and an object of the present invention is to provide an in-line optical isolator in which the insertion loss and isolation degradation are reduced even when ASE returns from the fiber amplifier.